A bonding master can be up while best_slave is NULL.

[12105.636318] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
[12105.638204] mlx4_en: eth1: Linkstate event 1 -> 1
[12105.648984] IP: bond_select_active_slave+0x125/0x250
[12105.653977] PGD 0 P4D 0
[12105.656572] Oops: 0000 [#1] SMP PTI
[12105.660487] gsmi: Log Shutdown Reason 0x03
[12105.664620] Modules linked in: kvm_intel loop act_mirred uhaul vfat fat stg_standard_ftl stg_megablocks stg_idt stg_hdi stg elephant_dev_num stg_idt_eeprom w1_therm wire i2c_mux_pca954x i2c_mux mlx4_i2c i2c_usb cdc_acm ehci_pci ehci_hcd i2c_iimc mlx4_en mlx4_ib ib_uverbs ib_core mlx4_core [last unloaded: kvm_intel]
[12105.685686] mlx4_core 0000:03:00.0: dispatching link up event for port 2
[12105.685700] mlx4_en: eth2: Linkstate event 2 -> 1
[12105.685700] mlx4_en: eth2: Link Up (linkstate)
[12105.724452] Workqueue: bond0 bond_mii_monitor
[12105.728854] RIP: 0010:bond_select_active_slave+0x125/0x250
[12105.734355] RSP: 0018:ffffaf146a81fd88 EFLAGS: 00010246
[12105.739637] RAX: 0000000000000003 RBX: ffff8c62b03c6900 RCX: 0000000000000000
[12105.746838] RDX: 0000000000000000 RSI: ffffaf146a81fd08 RDI: ffff8c62b03c6000
[12105.754054] RBP: ffffaf146a81fdb8 R08: 0000000000000001 R09: ffff8c517d387600
[12105.761299] R10: 00000000001075d9 R11: ffffffffaceba92f R12: 0000000000000000
[12105.768553] R13: ffff8c8240ae4800 R14: 0000000000000000 R15: 0000000000000000
[12105.775748] FS:  0000000000000000(0000) GS:ffff8c62bfa40000(0000) knlGS:0000000000000000
[12105.783892] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[12105.789716] CR2: 0000000000000000 CR3: 0000000d0520e001 CR4: 00000000001626f0
[12105.796976] Call Trace:
[12105.799446]  [<ffffffffac31d387>] bond_mii_monitor+0x497/0x6f0
[12105.805317]  [<ffffffffabd42643>] process_one_work+0x143/0x370
[12105.811225]  [<ffffffffabd42c7a>] worker_thread+0x4a/0x360
[12105.816761]  [<ffffffffabd48bc5>] kthread+0x105/0x140
[12105.821865]  [<ffffffffabd42c30>] ? rescuer_thread+0x380/0x380
[12105.827757]  [<ffffffffabd48ac0>] ? kthread_associate_blkcg+0xc0/0xc0
[12105.834266]  [<ffffffffac600241>] ret_from_fork+0x51/0x60

Fixes: e2a7420d ("bonding/main: convert to using slave printk macros")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: John Sperbeck <jsperbeck@google.com>
Cc: Jarod Wilson <jarod@redhat.com>
CC: Jay Vosburgh <j.vosburgh@gmail.com>
CC: Veaceslav Falico <vfalico@gmail.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: David S. Miller <davem@davemloft.net>